Changing the photoelectric conversion time (or change accumulation time) is the most common method to expand the dynamic range of a solid state image sensor. When the incoming light is strong, the charge accumulation time is shortened, and when the light is weak, the accumulation time (or the exposure time) is extended. However, when a picture including both a bright area (highlight) and a dark area (shadow) is to be taken by an image sensor, the proper accumulation time differs in those areas. When the accumulation time is adjusted for the highlight, the image data in the shadow cannot be obtained, and when the accumulation time is adjusted for the shadow, the photosensors in the highlight area saturate (i.e. the image data is all white or the photosensors will be destroyed). The solution to this problem is to expand the dynamic range of every pixel photosensor constituting the image sensor.
One of the measures for expanding the dynamic range of a solid state photosensor is described in the Japanese Patent Laid-open No. S6281183 where the voltage applied on the exit gate electrode of the charge storage is changed stepwise.